Current source devices are often used in high precision and high speed designs to drive virtually any type of electrical circuit. One such current source device is a current mirror circuit. The current mirror circuit includes a current source connected to a current reference device on the input leg. The output leg includes a current source device (e.g., a current source transistor) connected to a load circuit. A predetermined bias current (IBIAS) is applied by the current source to the current reference device that is mirrored to the current source device. The goal of the current mirror circuit is to provide matched currents in the input and output legs to drive the load circuit with the predetermined bias current supplied by the current source.
The current source device has a voltage-current characteristic of VDS to ID that has a corresponding family of VGS curves when the voltage applied by the current reference device to the current source device exceeds the threshold voltage, VT, plus a gate overdrive voltage. For a given VGS curve, the current source device will operate at a relatively constant current, ID, when the VDS of the current source device is greater than its saturation voltage, VDSAT. The amount that VDS is above VDSAT is known as the voltage margin. For the current reference device, the voltage margin is equal to the threshold voltage. However, because the load voltage imposed by the load circuit can vary, the VDS of the current source device will vary. The change in VDS of the current source device changes its output current. Hence, the current in the output leg will differ from the current of the input leg. Because the load circuit is often designed to operate with a particular current, or a particular range of currents, variations in the current in the output leg can cause the load circuit to malfunction.
Cascode devices are often used to overcome the problems associated with the mismatched currents in the input and output legs of the current mirror circuit. A typical conventional cascode current mirror circuit includes a current mirror circuit as described above with a cascode bias device on the input leg and a cascode device on the output leg. The cascode bias device forward biases the VGS of the cascode device. The cascode device establishes a current control voltage between the cascode device and the current source device that prevents the load voltage from significantly affecting the VDS of current source device. The result is that the currents in the input and output legs will be approximately equal.
However, because the threshold voltage, VT, utilized by the current mirror circuit is very large, e.g., about 700 mV, the conventional cascode current mirror circuit generates an excessive voltage margin, which wastes valuable headroom. Moreover, as temperature varies, the current control voltage does not track the saturation voltage of the current source device. Hence, the cascode current mirror is constrained by the worst-case margin that occurs at higher temperatures.
A conventional improved cascode current mirror circuit attempts to overcome the problems associated with the excessive voltage margin generated when the threshold voltage is utilized to enable the current source device. The conventional improved cascode current mirror circuit relies on adding a third leg to the conventional cascode current mirror circuit which utilizes a cascode bias device to replicate the VGS of the cascode device and a second cascode bias device which attempts to replicate the VDSAT plus a reduced fixed margin of the current source device. The solution offered by the conventional improved cascode current mirror however is not fully satisfactory because it relies on adjusting both the offset and the slope of the current control voltage at the same time (e.g., one “control knob”) in an attempt to track the current control voltage with the VDSAT of the current source device. Thus, one can not optimize the current control voltage with respect to each of the offset and the slope. If the current control voltage is optimized with respect to one the other is sacrificed and so a compromise adjustment must be settled on. The conventional improved cascode current mirror also does not accurately track the voltage margin of the current control voltage to the saturation voltage as temperature varies.